1. Field of the Invention
The present invention relates to an opto-electric conversion device with a new structure, or a light-receiving device.
2. Description of the Related Art
A light-receiving device has been known to have a pin junction structure. A backward voltage is applied to the pin layers of the device, and electron-hole pairs are generated by that light incided from the side of a p-layer is absorbed in an i-layer. The electron-hole pairs excited in the i-layer are accelerated by a backward voltage in the i-layer, and electrons and holes are flowing into an n-layer and a p-layer, respectively. Thus a photocurrent whose intensity varies according to an intensity of the incident light is outputted.
To improve an opto-electric conversion effectivity, the i-layer which absorbs light is formed to have a comparatively larger thickness. But when the thickness of the i-layer becomes thicker, more times are needed to draw carriers to the n-layer and the p-layer. As a result, the response velocity of the opto-electric conversion is lowered. To improve the velocity, an electric field in the i-layer is increased by increasing a backward voltage. But when the backward voltage is enlarged, an element separation become difficult and a leakage current is occurred. As a result, an photocurrent which flows when the device is not incided by light, or a dark current, is increased.
Thus conventional light-receiving devices had an interrelation among a light-receiving sensitivity, a detecting velocity, and a noise current, which restricts their performances.
It is, therefore, an object of the present invention to improve the light-receiving sensitivity and the response velocity of the opto-electric conversion by providing a light-receiving device having a pin junction of a completely new structure.
In light of these objects a first aspect of the present invention is a light-receiving device, which converts incident light into electric current, constituted by a quantum-wave interference layer units having plural periods of a pair of a first layer and a second layer, the second layer having a wider band gap than the first layer, and a carrier accumulation layer disposed between adjacent two of the quantum-wave interference layer units. Each thickness of the first and the second layers is determined by multiplying by an even number one fourth of a quantum-wave wavelength of carriers in each of the first and the second layers, and the carrier accumulation layer has a band gap narrower than that of said second layer. Plural units of the quantum-wave interference layers are formed with a carrier accumulation layer, which has a band gap narrower than that of the second layer, lying between each of the quantum-wave interference units.
The second aspect of the present invention is to set a kinetic energy of the carriers, which determines the quantum-wave wavelength, at the level near the bottom of a conduction band when the carriers are electrons or at the level near the bottom of a valence band in the second layer when the carriers are holes.
The third aspect of the present invention is to define each thickness of the first and the second layers as follows:
DW=nWxcexW/4=nWh/4[2mW(E+V)]1/2xe2x80x83xe2x80x83(1)
and
DB=nBxcexB/4=nBh/4(2mBE)1/2xe2x80x83xe2x80x83(2)
In Eqs. 1 and 2, h, mW, mB, E, V, and nW, nB represent Plank""s constant, the effective mass of carrier conducting in the first layer, the effective mass of carriers in the second layer, the kinetic energy of the carriers at the level near the lowest energy level of the second layer, the potential energy of the second layer relative to the first layer, and even numbers, respectively.
The fourth aspect of the present invention is a quantum-wave interference layer having a partial quantum-wave interference layers Ik with arbitrary periods Tk including a first layer having a thickness of nWkxcexWk/4 and a second layer having a thickness of nBkxcexBk/4 for each of a plural different values Ek, Ek+V. Ek, Ek+V, xcexBk,xcexWk, and nBk, nWk represent a kinetic energy of carriers conducted in the second layer, a kinetic energy of carriers conducted in the first layer, a quantum-wave wavelength corresponding energies of the second layer and the first layer, and even numbers, respectively.
The fifth aspect of the present invention is to form a carrier accumulation layer having the same bandwidth as that of the first layer.
The sixth aspect of the present invention is to form a carrier accumulation layer having a thickness same as its quantum-wave wavelength xcexW.
The seventh aspect of the present invention is to form a xcex4 layer between the first layer and the second layer, which sharply varies band gap energy at the boundary between the first and second layers and is substantially thinner than that of the first and the second layers.
The eighth aspect of the present invention is a light-receiving device having a pin junction structure, and the quantum-wave interference layer and the carrier accumulation layer are formed in the i-layer.
The ninth aspect of the present invention is to form the quantum-wave interference layer and the carrier accumulation layer in the n-layer or the p-layer.
The tenth aspect of the present invention is a light-receiving device having a pin junction structure.
First to Third, and Eighth to Tenth Aspects of the Invention
The principle of the light-receiving device of the present invention is explained hereinafter. FIG. 1 shows an energy diagram of a conduction band and a valence band when an external voltage is applied to the interval between the p-layer and the n-layer in a forward direction. As shown in FIG. 1, the conduction band of the i-layer becomes plane by applying the external voltage. Four quantum-wave interference layer units Q1 to Q4 are formed in the i-layer, and carrier accumulation layers C1 to C3 are formed at each intervals of the quantum-wave interference layer units. FIG. 2 shows a conduction band of a quantum-wave interference layer unit Q1 having a multi-layer structure with plural periods of a first layer W and a second layer B as a unit. A band gap of the second layer B is wider than that of the first layer W.
Electrons conduct from left to right as shown by an arrow in FIG. 2. Among the electrons, those that exist at the level near the lowest energy level of a conduction band in the second layer B are most likely to contribute to conduction. The electrons near the bottom of conduction band of the second layer B has a kinetic energy E. Accordingly, the electrons in the first layer W have a kinetic energy E+V which is accelerated by potential energy V due to the band gap between the first layer W and the second layer B. In other words, electrons that move from the first layer W to the second layer B are decelerated by potential energy V and return to the original kinetic energy E in the second layer B. As explained above, kinetic energy of electrons in the conduction band is modulated by potential energy due to the multi-layer structure.
When thicknesses of the first layer W and the second layer B are equal to order of quantum-wave wavelength, electrons tend to have characteristics of a wave. The wave length of the electron quantum-wave is calculated by Eqs. 1 and 2 using kinetic energy of the electron. Further, defining the respective wave number vector of first layer W and second layer B as KW and KB, reflectivity R of the wave is calculated by:                                                                         R                =                                                      (                                          |                                              K                        W                                            |                                              -                                                  |                                                      K                            B                                                    |                                                                                      )                                    /                                      (                                          |                                              K                        W                                            |                                              +                                                  |                                                      K                            B                                                    |                                                                                      )                                                                                                                          =                                                      (                                                                                            [                                                                                    m                              W                                                        ⁡                                                          (                                                              E                                +                                V                                                            )                                                                                ]                                                                          1                          /                          2                                                                    -                                                                        [                                                                                    m                              B                                                        ⁢                            E                                                    ]                                                                          1                          /                          2                                                                                      )                                    /                                                            (                                                                                                    [                                                                                          m                                W                                                            ⁡                                                              (                                                                  E                                  +                                  V                                                                )                                                                                      ]                                                                                1                            /                            2                                                                          +                                                                              [                                                                                          m                                B                                                            ⁢                              E                                                        ]                                                                                1                            /                            2                                                                                              )                                        .                                                                                                                          =                                                      [                                          1                      -                                                                        (                                                                                    m                              B                                                        ⁢                                                          E                              /                                                                                                m                                  W                                                                ⁡                                                                  (                                                                      E                                    +                                    V                                                                    )                                                                                                                                              )                                                                          1                          /                          2                                                                                      ]                                    /                                      [                                          1                      +                                                                        (                                                                                    m                              B                                                        ⁢                                                          E                              /                                                                                                m                                  W                                                                ⁡                                                                  (                                                                      E                                    +                                    V                                                                    )                                                                                                                                              )                                                                          1                          /                          2                                                                                      ]                                                                                      "AutoLeftMatch"                            (        3        )            
Further, when mB=mW, the reflectivity R is calculated by:
R=[1xe2x88x92(E/(E+V))1/2]/[1+(E/(E+V))1/2]xe2x80x83xe2x80x83(4).
When E/(E+V)=x, Eq. 4 is transformed into:
R=(1xe2x88x92x1/2)/(1+x1/2)xe2x80x83xe2x80x83(5).
The characteristic of the reflectivity R with respect to the energy ratio x obtained by Eq. 5 is shown in FIG. 3.
When the condition xxe2x89xa6{fraction (1/10)} is satisfied, Rxe2x89xa70.52. Accordingly, the relation between E and V is satisfied with:
Exe2x89xa6V/9xe2x80x83xe2x80x83(6).
Since the kinetic energy E of the conducting electrons in the second layer B exists near the bottom of the conduction band, the relation of Eq. 6 is satisfied and the reflectivity R at the interface between the second layer B and the first layer W becomes 52% or more. Consequently, the multi-layer structure having two kinds of layers with band gaps different from each other enables to reflect quantum-wave of electrons which is injected to an i-layer.
Further, utilizing the energy ratio x enables the thickness ratio DB/DW of the second layer B to the first layer W to be obtained by:
DB/DW=[mW/(mBx)]1/2xe2x80x83xe2x80x83(7).
When thicknesses of the first and second layers are determined by multiplying an even number by one fourth of a quantum-wave wavelength, or by a half of a quantum-wave wavelength, for example, a standing wave rises in a quantum-wave interference layer, and a resonant conduction is occurred. That is, when a quantum-wave period of the standing wave and a potential period of the quantum-wave interference layer is corresponded to each other, a scattering of the carrier in each layer is suppressed, and a conduction of a high mobility is realized.
When light is incided to the i-layer formed in the light-receiving device, electrons excited in conduction bands of the carrier accumulation layers C1, C2 and C3 are accumulated therein. The excited electrons tend to flow to the p-layer by the applied forward voltage. But the energy which the excited electrode have is lower than the bottom of the conduction band in the second layer B. Accordingly, the electrons do not flow because a transmission condition is not satisfied for electrons in the quantum-wave interference layer unit which exists at the side toward the p-layer.
But when the electrons existing in the carrier accumulation layers C1, C2 and C3 are increased, electrons tend to exist in higher level. Then a kinetic energy of the electrons existing in higher level increases, and the electrons can highly conduct or transmit in the quantum-wave interference layer units because of satisfaction of the transmission condition. As a result, the electrons passes the quantum-wave interference layer units Q2, Q3, and Q4 and flow toward the p-layer, which occurs a photocurrent.
Because a forward voltage is applied to the light-receiving device, driving at a low voltage becomes possible and an element separation become easier. When light is not incided, electrons does not have a high transmittivity in the quantum-wave interference layer units. As a result, a dark current can be lowered. The present inventor thinks that electrons is conducted in the quantum-wave interference layer units as a wave. Accordingly, a response velocity is considered to become larger.
Thicknesses of the first layer W and the second layer B are determined for selectively transmitting one of holes and electrons, because of a difference in potential energy V between the valence and the conduction bands, and a difference in effective mass of holes and electrons in the first layer W and the second layer B. Namely, the optimum thickness of the first and the second layers for transmitting electrons is not optimum for transmitting holes. Eqs. 5-9 refer to a structure of the quantum-wave interference layer for transmitting electrons selectively. The thickness for selectively transmitting electrons is designed based on the potential difference in the conduction band and effective mass of electrons. Consequently, the quantum-wave interference layer has a high transmittivity (or a high mobility) for electrons, but not for holes.
Further, the thickness for selectively transmitting holes is designed based on a difference in potential energy of the valence band and effective mass of holes, realizing another type of quantum-wave interference layer as a hole transmission layer, which has a high mobility for holes and which has an ordinary mobility for electrons.
Further explanation can be obtained by FIGS. 4A-4H. FIGS. 4A-4H illustrate the relationship between quantum-wave reflection of electrons in a potential of quantum-well structure and a period of potential representing a conduction band of a multi quantum-well (MQW). FIGS. 4A-4D show the relationship when the period, i.e., width of the second layer B or the first layer W, of the potential is equal to an odd number multiplied by one fourth of the wavelength of propagated electron. This type of the potential is named as xcex/4 type potential hereinafter. FIGS. 4E-4H show when the period of the potential is equal to a natural number multiplied by a half of the wavelength of propagated electron. This type of the potential is named as xcex/2 type potential hereinafter. In order to make it visually intelligible, thickness of each layers is unified in FIGS. 4A-4H. Electrons existing around the bottom of the second layer B conduct from left to right as shown by an arrow in FIGS. 4A and 4E. And in FIGS. 4B and 4F, the electrons reach the interface between the first layer W and the second layer B.
When the quantum-wave of the electrons reaches the interface between the second layer B and the first layer W in the xcex/4 type potential, a transmission wave QW2 and a reflection wave QW3 having a phase equal to that of the transmission wave QW2, are generated with respect to an incident wave QW1 as shown in FIG. 4C. Then when the transmission wave QW2 reaches the interface between the first layer W and the second layer B, a transmission wave QW4 and a reflection wave QW5 having a phase opposite to that of the transmission wave QW4 are generated as shown in FIG. 4D. The relationship between phases of the transmission wave and the reflection wave at the interface depends on following or rising of a potential of the conduction band at the interface. In order to make it visually intelligible, each amplitudes of QW1, QW2, QW3, QW4, and QW5 is unified in FIGS. 4A-4H.
With respect to the xcex/4 type potential of the multi quantum-well, the propagating quantum-wave of electrons represented by QW1, QW2 and QW4 and the reflecting quantum-wave of electrons represented by QW3 and QW5 cancels with each other, as shown in FIG. 4D. The quantum-wave of electrons represented by the QW1, QW2 and QW4 propagates from left to right, and the quantum-wave of electrons represented by the QW3 and QW5, generated by the reflection at two interfaces, propagates from right to left. Accordingly, a multi quantum-well, having a potential which is formed in a period, i.e., the width of the first layer W and the second layer B, determined by multiplying by an odd number one fourth of quantum-wave wavelength of propagated electrons, cancels the quantum-wave of electrons. In short, the multi quantum-well functions as a reflection layer which does not propagate electrons.
With respect to a multi quantum-well, having a potential which is formed in a period, i.e., the width of the first layer W and the second layer B, determined by multiplying by an even number one fourth of quantum-wave wavelength of propagated electrons, i.e., xcex/2 type potential, as shown in FIGS. 4E-4H, the quantum-wave of electrons can become a standing wave.
Similarly, when a quantum-wave of electrons reaches the interface between the second layer B and the first layer W in the xcex/2 type potential, a transmission wave QW2 and a reflection wave QW3 having a phase corresponding to that of the transmission wave QW2, are generated with respect to an incident wave QW1 as shown in FIG. 4G. Then when the transmission wave QW2 reaches the interface between the first layer W and the second layer B, a transmission wave QW4 and a reflection wave QW5 having a phase opposite to that of the transmission wave QW4 are generated as shown in FIG. 4H. With respect to xcex/2 type potential of the multi quantum-well, the propagating quantum-wave of electrons represented by QW1, QW2 and QW4 and the reflecting quantum-wave of electrons represented by QW5 intensifies to each other, as shown in FIG. 4H. On the other hand, the reflection waves QW3 and QW5 can be considered to cancel with each other and the quantum-wave of electrons which-is propagated from left to right in FIG. 4E can be a standing wave. Accordingly, with respect to the multi quantum-well, having a potential which is formed in a period, i.e., the width of the first layer W and the second layer B, determined by multiplying by an even number one fourth of quantum-wave wavelength of propagated electrons, the quantum-wave of electrons can become a standing wave and a transmission layer having a high transmittivity (or a high mobility) for electrons can be realized.
Alternatively, a multi quantum-well, having a potential which is formed in a period determined by multiplying by a natural number half of quantum-wave wavelength of holes, can be applied to the relationship described above.
The quantum-wave interference layer unit described above can transmit carriers in accordance with numbers of electrons accumulated in the carrier accumulation layer. Accordingly, the light-receiving device can be formed by only one of the n-layer and the p-layer in which the quantum-wave interference layer units and the carrier accumulation layer are formed. Alternatively, the light-receiving device can be formed by a pn junction structure, in which the quantum-wave interference layer units and the carrier accumulation layer are formed in at least one of n-layer and p-layer.
Fourth Aspect of the Present Invention
FIG. 5 shows a plurality quantum-wave interference units Ik with arbitrary periods Tk including a first layer having a thickness of DWk and a second layer having a thickness of DBk and arranged in series.
Each thickness of the first and the second layers satisfies the formulas:
DWk=nWkxcexWk/4=nWkh/4[2mWk(Ek+V)]1/2xe2x80x83xe2x80x83(8)
and
DBk=nBkxcexBk/4=nBkh/4(2mBkEk)1/2xe2x80x83xe2x80x83(9)
In Eqs. 8 and 9, Ek, mWk, mBk, and nWk and nBk represent plural kinetic energy levels of carriers conducted into the second layer, effective mass of carriers with kinetic energy Ek+V in the first layer, effective mass of carriers with kinetic energy Ek in the second layer, and arbitrary even numbers, respectively.
The plurality of the partial quantum-wave interference layers Ik are arranged in series from I1 to Ij, where j is a maximum number of k required to form a quantum-wave interference layer as a whole. The carriers existing in a certain consecutive energy range can be effectively transmitted by narrowing a discrete intervals.
Fifth and Sixth Aspects of the Present Invention
The fifth aspect of the present invention is to form the band width of the carrier accumulation layer to have the same bandwidth as that of the first layer. And the sixth aspect of the present invention is to form the carrier accumulation layer to have a thickness same as its quantum-wave wavelength xcexW. As a result, the carriers excited in the carrier accumulation layer can be confined effectively.
Seventh Aspect of the Present Invention
The seventh aspect of the present invention is directed forming a xcex4 layer at the interface between the first layer W and the second layer B. The xcex4 layer has a relatively thinner thickness than both of the first layer W and the second layer B and sharply varies an energy band. By sharply varying the band gap of the interfaces, the potential energy V of an energy band becomes larger substantially and the value x of Eq. 5 becomes smaller, as shown in FIGS. 7A-7D. Without forming a xcex4 layer as shown in FIG. 7A, a part of component of the first layer W and the second layer B mixes when the second layer B is laminated on the first layer W, and an energy band gap which varies sharply cannot be obtained, as shown in FIG. 7B. When a xcex4 layer is formed at each interfaces of the first and the second layers, as shown in FIG. 7C, even if a part of component of the first layer W and the second layer B mixes, an energy band gap varies sharply compared with the case without xcex4 layers, as shown in FIG. 7D.
Variations are shown in FIGS. 6A to 6D. The xcex4 layer may be formed on both ends of the every first layer W as shown in FIGS. 6A to 6D. In FIG. 6A, the xcex4 layers are formed so that an energy level higher than that of the second layer B may be formed. In FIG. 6B, the xcex4 layers are formed so that a band having lower bottom than that of the first layer W may be formed. In FIG. 6C, the xcex4 layers are formed so that the energy level higher than that of the second layer B and the energy level lower than that of the first layer W may be formed. As an alternative to each of the variations shown in FIGS. 6A to 6C, the xcex4 layer can be formed on one end of the every first layer W as shown in FIG. 6D.